VCSEL mode-transforming phase filter with enhanced performance

ABSTRACT

A vertical cavity surface emitting laser (VCSEL) in which a higher order lasing mode produces a Gaussian-like single mode far field beam intensity pattern. Such a VCSEL includes a protective surface deposition on a VCSEL structure, and phase filter elements on the surface deposition. The surface deposition and the phase filter elements implement an optical phase filter that induces optical path difference such that a single mode far field beam intensity pattern results when the VCSEL operates in a higher order lasing mode. The VCSEL can include structures that enhance a selected higher-order operating mode and/or that suppress unwanted operating modes.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Not Applicable.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to vertical cavity surface emittinglasers. More specifically, this invention relates to using higher orderlasing modes in vertical cavity surface emitting lasers.

[0004] 2. Discussion of the Related Art

[0005] Vertical cavity surface emitting lasers (VCSELs) represent arelatively new class of semiconductor lasers. While there are manyvariations of VCSELs, one common characteristic is that they emit lightperpendicular to a wafer's surface. Advantageously, VCSELs can be formedfrom a wide range of material systems to produce specificcharacteristics. In particular, the various material systems can betailored to produce different laser wavelengths, such as 1550 nm, 1310nm, 850 nm, 670 nm, and so on. In general, VCSELs include semiconductoractive regions, distributed Bragg reflector (DBR) mirrors, currentconfinement structures, substrates, and electrical contacts.

[0006]FIG. 1 illustrates a typical VCSEL 10. As shown, an n-dopedgallium arsenide (GaAs) substrate 12 has an n-type electrical contact14. An n-doped lower mirror stack 16 (a DBR) is on the GaAs substrate12, and an n-type graded-index lower spacer 18 is disposed over thelower mirror stack 16. An active region 20, usually having a number ofquantum wells, is formed over the lower spacer 18. A p-type graded-indextop spacer 22 is disposed over the active region 20, and a p-type topmirror stack 24 (another DBR) is disposed over the top spacer 22. Overthe top mirror stack 24 is a p-type conduction layer 9, a p-type GaAscap layer 8, and a metallic electrical contact 26.

[0007] Still referring to FIG. 1, the lower spacer 18 and the top spacer22 separate the lower mirror stack 16 from the top mirror stack 24 suchthat an optical cavity is formed. As the optical cavity is resonant atspecific wavelengths, the mirror separation is controlled to resonate ata predetermined wavelength (or at a multiple thereof). At least part ofthe top mirror stack 24 includes an insulating region 40 that is usuallyformed either by implanting protons into the top mirror stack 24 or byproviding an oxide layer. The oxide layer can be formed, for example, inaccordance with the teachings of U.S. Pat. No. 5,903,588, which isincorporated by reference. The insulating region 40 defines a conductiveannular central opening 42. Thus, the central opening 42 forms anelectrically conductive path through the insulating region 40.

[0008] In operation, an external bias causes an electrical current 21 toflow from the p-type electrical contact 26 toward the n-type electricalcontact 14. The insulating region 40 and the conductive central opening42 confine the current 21 such that it flows through the conductivecentral opening 42 into the active region 20. Some of the electrons inthe current 21 are converted into photons in the active region 20. Thosephotons bounce back and forth (resonate) between the lower mirror stack16 and the top mirror stack 24. While the lower mirror stack 16 and thetop mirror stack 24 are very good reflectors, some of the photons leakout as light 23 that travels along an optical path. Still referring toFIG. 1, the light 23 passes through the p-type conduction layer 9,through the p-type GaAs cap layer 8, through an aperture 30 in themetallic electrical contact 26, and out of the surface of the verticalcavity surface emitting laser 10.

[0009] It should be understood that FIG. 1 illustrates a common VCSELstructure, and that numerous variations are possible. For example, thedopings can be changed (say, by providing a p-type substrate 12),different material systems can be used, operational details can be tunedfor maximum performance, and additional structures, such as tunneljunctions, can be added.

[0010] While generally successful, VCSELs have problems. In particular,some applications require a high power laser source that produces aGaussian-like beam intensity distribution in the far field.Unfortunately, VCSELs can and do support a large number of higher orderlasing modes, conventionally designated in the LP approximation as m,lmodes. In particular, modes with m>0, and with l=1, that produce “donut”or “necklace” or “flower-petal” beam intensity distributions that do nothave the required far field properties. On the other hand, otherapplications, such as high-speed data communication over multi-modeoptical fiber, often achieve best performance if the far field intensityis derived from a specific higher-order “necklace” mode.

[0011] One method of obtaining the desired Gaussian-like beam intensitydistribution is to reduce the diameter of the active region. While thisis successful in quenching higher order modes, a small diameter activeregion results in low output power, a large series resistance, and tightfabrication tolerances.

[0012] Fourier optics provides for phase-shift apertures that can filterand/or tailor the far-field intensity distributions of an incidentfield. Recent art has demonstrated the use of a phase filter in theaperture of a VCSEL that tailors a higher-order “necklace” incident modeinto a far-field beam intensity distribution that appears Gaussian-like.For example, S. Shinada et al. in “Far Field Pattern Control of SingleHigh Order Transverse Mode VCSEL with Micromachined Surface Relief”discloses the use of micromachined top surfaces layers that producealternating π-phase shifts. The alternating π-phase shift layers canproduce a reasonable Gaussian-like single-mode far field pattern.

[0013] While S. Shinada et al. disclose a technically interestingconcept, their approach is less than optimal for many practicalapplications. For example, S. Shinada et al. use a focused ion beam toetch the top layer of the VCSEL to form the alternating π-phase shiftlayers. This can induce damage on the VCSEL itself. Furthermore, it isdifficult to accurately control the phase shifts of the individuallayers to achieve optically neutral slices. That is, a slice that doesnot modulate the reflection, as seen from the cavity, or thetransmission, as seen from the far field.

[0014] Therefore, an improved Fourier optical system that produces asingle-mode far field intensity distribution pattern from a higher orderlasing VCSEL would be beneficial. Also beneficial would be a VCSELhaving an optical phase filter that forms a Gaussian-like single modefar field pattern from a higher order lasing mode, with the opticalphase filter implemented by a surface deposition of pie-slice filterelements that not only produce suitable optical path length differencesto achieve the far field distribution, but that also protect the surfaceof the VCSEL. Even more beneficial would be a VCSEL having an opticalphase filter that forms a Gaussian-like single mode far field beamintensity pattern from a selected higher order lasing mode, with theVCSEL implemented in a manner that promotes the selected higher orderlasing mode. Also beneficial would be a VCSEL having an optical phasefilter that forms a single mode far field beam intensity pattern from aselected higher order lasing mode, with optical structures that promotethe selected higher order lasing mode, and with electrical currentconfinement that produces current flow into the active region that tendsto enhance the selected higher order lasing mode. Further, in somecircumstances it is beneficial to have a far field intensity resemblinga specific higher-order mode pattern that is created by converting alower-order lasing mode. Thus, the optical phase filter is preferablyused in conjunction with other techniques that tend to pin the operatingmode of the VCSEL to a selected mode.

SUMMARY OF THE INVENTION

[0015] Accordingly, the principles of the present invention are directedto a new VCSEL design in which a higher order lasing mode produces aGaussian-like single mode far field beam intensity pattern.

[0016] The principles of the present invention are also directed to anew VCSEL design in which a lower-order lasing mode produces anintensity distribution resembling a specific high-order mode pattern inthe far field.

[0017] The principles of the present invention extend to techniques thatprotect the VCSEL and to techniques that improve the available opticalpower of intensity distributions in the far field, the intensitydistributions being created either by converting a lower-order lasingmode to a higher-order lasing mode or by converting a higher-orderlasing mode to a lower-order lasing mode.

[0018] A VCSEL according to the present invention is comprised of asurface deposition on a VCSEL structure, and of phase filter elementsthat are fabricated on the surface deposition. The phase filter elementsimplement an optical phase filter that induces optical path lengthdifference between adjacent phase filter elements such that (1) aGaussian-like, single mode far field beam intensity pattern results whenthe VCSEL operates with a higher order lasing mode, or (2) an intensitydistribution resembling a specific high-order mode pattern in the farfield results when the VCSEL operates with a lower-order lasing mode.Beneficially, the phase filter elements are comprised of differentmaterials, with the thickness of one material being dependent on thefirst material. Also beneficially, the VCSEL is protected by the surfacedeposition. With a specific configuration of the phase filter elements,for example, by adding additional phase shift levels, the optical powerof the desired far-field intensity distribution can also besignificantly increased.

[0019] Preferably, the phase filter elements are used in conjunctionwith a structure or with structures that tend to pin the VCSEL operatingmode at a selected operating mode. For example, metal contact fingerscan be added to ohmic contact metal such that the metal fingers createcurrent distributions in the active region so as to support a selectedoperating mode. Alternatively, implants can be added to the top mirrorstructure to tailor the conductive paths to those that support theselected operating mode.

[0020] Another set of structures that tend to pin the VCSEL at aselected operating mode act by suppressing unwanted VCSEL modes. Forexample, optical losses for unwanted operating modes can be increased byincorporating antireflective zones, thin scattering zones, or absorptiveregions that are located at the null field(s) of the selected operatingmode. Such structures tend to quench lasing of the unwanted opticalmodes but do not significantly impact the selected operating mode.Another approach is to suppress the fundamental and lower order modes byadding “inverse fetch” regions (zones of lower reflectivity), highresistance regions, or optical absorptive structures near the centralaxis.

[0021] Yet another set of structures that pin the VCSEL to a selectedoperating mode is oxide-confined cavities that are located near theactive region and that produce an effective confining index differencesuch that the selected operating mode is operationally preferred.Beneficially, large oxide apertures should be used.

[0022] Additional features and advantages of the invention will be setforth in the description that follows, and in part will be apparent fromthat description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWING

[0023] The accompanying drawings, which are included to provide afurther understanding of the invention and which are incorporated in andconstitute a part of this specification, illustrate various embodimentsof the invention and together with the description serve to explain theprinciples of the invention.

[0024] In the drawings:

[0025]FIG. 1 illustrates a typical VCSEL;

[0026]FIG. 2 illustrates a schematic plan view of a VCSEL that is inaccord with the principles of the present invention;

[0027]FIG. 3 illustrates a schematic, isometric view of the VCSEL ofFIG. 2;

[0028]FIG. 4 illustrates a schematic plan view of a VCSEL having ohmiccontact metal fingers and an oxidized film for mode confinement;

[0029]FIG. 5 illustrates a schematic plan view of a VCSEL having phaseshift filters configured for the blazing technique;

[0030]FIG. 6 illustrates a schematic plan view of a VCSEL having ohmicmetal contact fingers, an oxide confinement cavity, and an inverse fetchstructure;

[0031]FIG. 7 illustrates a VCSEL having various techniques that tend tosuppress the fundamental and lower order operating modes; and

[0032]FIG. 8 illustrates a VCSEL having various techniques that tend toenhance a specific higher-order operating mode.

[0033] Note that in the drawings that like numbers designate likeelements. Additionally, for explanatory convenience the descriptions usedirectional signals such as up and down, top and bottom, and lower andupper. Such signals, which are derived from the relative positions ofthe elements illustrated in the drawings, are meant to aid theunderstanding of the present invention, not to limit it.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

[0034] One of the principles of the present invention provides forVCSELs having optical phase filters for converting a near fieldhigher-order mode operation to a far field Gaussian-like single modeoperation. Another principle of the present invention provides forVCSELs having optical phase filters for converting a near fieldlower-order mode operation to an intensity distribution resembling aspecific high-order mode pattern in the far field. Those principlesfurther include the use of a structure or structure(s) that tend to pinthe operating mode of the VCSEL to a selected mode or that tend tosuppress unwanted operating modes.

[0035] The previously mentioned disclosure of S. Shinada et al., “FarField Pattern Control of Single High Order Transverse Mode VCSEL withMicromachined Surface Relief,” discussed π-phase shift layers formed bymicromachining a surface of a VCSEL. Shinada et al. phase shift layersresembles a pie having an even number of equally sized slices, withadjacent slices inducing alternating phases shifts of 0 and π. Inactuality, adjacent slices could produce phase shifts of 2Nπ and(2N+1)π, where N=0, 1, 2, 3, . . .

[0036] Shinada et al. disclose depositing a phase-shifting dielectriclayer, such as SiO₂ or Si₃N₄, on etched “pie-slice” shaped VCSELstructures that are separated by trenches. The film thickness andmaterials, as well as the etch depth must be controlled to produce anoptical path difference of π/2 between adjacent VCSEL structures.Implementing such a VCSEL can damage the VCSEL structures and requiresdifficult process controls.

[0037] A far field analysis of the Shinada et al. VCSEL structure willbe useful in what follows. Since a VCSEL commonly exhibits cylindricalsymmetry it is best described in cylindrical coordinates (r, φ, z). Thephysical mechanisms that lead to mode confinement in a VCSEL, such asrefractive index distribution, generally depend only on the radialcoordinate r and are independent of the azimuthal angle φ. Thus, themodes supported by a VCSEL can be designated by two indices—a radialindex 1 and an azimuthal index m—and have the following generalfunctional form:

F _(ml)(r,φ)=G _(ml)(r)exp(imφ).  (1)

[0038] The function describing the radial dependence, G_(ml)(r), isdetermined by the radial variation in the refractive index. For example,if the refractive index is a step function having a value n_(core) inthe aperture and a smaller value n_(clad) outside the aperture, thenG_(ml)(r) are Bessel functions (J_(m)(r) in the aperture and K_(m)(r)outside the aperture).

[0039] When the field described by equation (1) is propagated to theVCSEL surface, (accounting for transmittance), and squared, it yieldsthe near-field intensity distribution for that mode. The angularintensity distribution in the far-field is found by multiplying thefield amplitude in (1) by an aperture transmittance (or phase) function,Fourier transforming, taking the real part, and squaring. Specifically,let M be the number of slices in the aperture phase filter (M must be aneven integer).

M=2L, L=1, 2, 3, . . .  (2)

[0040] The transmittance (phase) function of the filter is then:

t _(M)(φ)=exp(iπ(n−1)) 2π(n−1)/M<φ<2πn/M, n=1, 2, 3, . . . , M−1, M  (3)

[0041] The far-field intensity (watt/sr) for mode l,m is then:

I _(ml)(θ,φ)=[

[FT{G _(ml)(r)exp(imφ)t _(M)(φ)}]]²,  (4)

[0042] where θ, φ denote the polar and azimuthal angles, respectively,and FT denotes the Fourier transform. The Fourier transform incylindrical coordinates, sometimes called a Hankel transform, isstraightforward to evaluate. The result is: $\begin{matrix}{{{S_{m\quad l}\left( {\theta,\varphi} \right)} = {\underset{n}{\Sigma}{c_{n}\left( {1/i} \right)}^{n}{\exp \left( {\quad n\quad \varphi} \right)}{\int_{aperture}^{\quad}{{G_{m\quad l}(r)}{J_{n}\left( {k_{r}r} \right)}r\quad {r}}}}},} & (5)\end{matrix}$

[0043] and c_(n) is the azimuthal coefficient: $\begin{matrix}{c_{n} = {\int_{0}^{2\quad \pi}{{t_{M}(\varphi)}{\exp \left( {\quad \left( {m - n} \right)\quad \varphi} \right)}\quad {{\varphi}.}}}} & (6)\end{matrix}$

[0044] The summation in n extends over all integers in the domain(−∞,∞). The polar angle θ can be expressed in terms of the radialcomponent of the wavevector:

k _(r)=(2π/λ)n _(SUIT) sin(θ),  (6)

[0045] where n_(SUIT) is the refractive index of the surrounding medium(usually air).

[0046] Generally, one finds a θ-dependence in the far-field intensityhaving a shape similar to the radial variation of the underlying VCSELmode. For now, it is sufficient to examine only the azimuthalcoefficients c_(n) in order to understand the behavior of the phasefilter.

[0047] Based on the foregoing, it can be shown that a phase filterdesign similar to that of Shinada et al. (M=8) will convert afundamental mode (m,l=0,1) VCSEL output into a far-field pattern thatlooks like a number of “donut” shaped intensities starting with amaximum amplitude at azimuthal index 4. Conversely, a VCSEL output of amode m=4 (which looks like a “necklace” having eight “pearls”) istransformed by the phase filter into two different modes in thefar-field, one of which looks like a fundamental mode and the other (ofequal amplitude) which looks like a 16-pearl necklace. The trend isclear, to produce a “fundamental-like” mode in the far-field the VCSELshould predominately lase in a selected higher-order mode with m=M/2.Thus, if a VCSEL lases predominately in a necklace mode with m=5, (10“pearls” in the near-field) then a 10 slice phase filter on the VCSELwill produce a “fundamental-like” mode in the far-field.

[0048] The principles of the present invention use the foregoing as abasis to provide new, useful, and nonobvious VCSELs having optical phasefilters that tend to produce either a single mode intensity distributionpattern in the far field when the VCSEL lases in a higher order mode oran intensity distribution resembling a specific high-order mode patternin the far field when the VCSEL lases in a lower-order lasing mode.Furthermore, such VCSELs can include a structure or structures that tendto suppress unwanted operating modes and/or that tend to pin the VCSELoperating mode to a selected operating mode that matches the opticalphase filter characteristics.

[0049]FIGS. 2 and 3 illustrate a first VCSEL 198 that incorporates theprinciples of the present invention. FIG. 2 schematically depicts asimplified top down view of the VCSEL 198 while FIG. 3 shows asimplified isometric view (without a metal contact). The VCSEL 198includes a metal contact 200 (not shown in FIG. 3 for clarity) thatintroduces current into an active region (reference, for example, theactive region 20 and the p-type conduction layer 9 of FIG. 1). The VCSEL198 includes an underlying (oxide or gain guide) aperture 202(reference, for example, the central opening 42 of FIG. 1) for applyingcurrent to the active region.

[0050] Turning now to FIG. 3, the bottom cylinder 206 of the VCSEL 198schematically represents the light-emitting portions of the VCSEL 198.Reference the top mirror stack 24, the p-type conduction layer 9, thep-type GaAs cap layer 8, the aperture 30, and the p-type electricalcontact 26 of the vertical cavity surface emitting laser 10 of FIG. 1.

[0051] Still referring to FIG. 3, over the bottom cylinder 206 is amiddle cylinder 208 that is comprised of a suitable optical material(e.g. a dielectric such as oxide or nitride, or a semiconductor materiallatticed-matched to the VCSEL) that shifts the phase of the emittedlaser light by a half-wavelength (λ/2). The middle cylinder 208physically protects the bottom cylinder 206 while providing an opticallyneutral base for a pie-slice phase filter that is comprised of pie-slicephase filter elements 210.

[0052] As shown in FIG. 3, the pie-slice phase filter is comprised ofequal numbers of interlaced pie-slice phase filter elements 210 and openareas 212. The pie-slice phase filter elements 210 and open areas 212are implemented to produce an optical path difference that is an integermultiple of π/2 between the optical path through the open areas 212 andthat through the pie-slice phase filter elements 210. It should bepointed out that it is the optical path difference that counts, not theopen areas. What is significant is that the optical path differencebetween adjacent slices (in FIG. 3, between an open area 212 and anadjacent phase filter elements 210) is (2m+1)(λ/2), where m=0, 1, 2, . .. . If air (refractive index of 1) is the medium M1 for one slice (anopen area 212), then the thickness of the adjacent slice (a phase filterelement 210) is:

d=(2m+1)(λ/2)/(n ₂−1),

[0053] where n₂ is the refractive index of the material M2 of the phasefilter element 210.

[0054] In practice, the simplest case is when m=0. Furthermore, alift-off process that removes the material M2 from the middle cylinder208 is probably the easiest to implement, but etch removal techniquesare also commonly used.

[0055] Ideally each phase filter element 210 is optically neutral suchthat it doesn't modulate the reflection (as seen from the cavity) or thetransmission (as seen from the far field) of the laser beam. This causesthe resulting structure to be a pure phase filter, which can be achievedby selecting the phase filter element 210 material M2 such that it has arefractive index of 1.5 or 2.0, i.e., a value such that (n₂−1)=0.5 or 1.Either SiO₂ (n=˜1.47) or Si₃N₄ (n=˜2.0) are suitable. Furthermore, TiO₂(n=˜2.5) is a good choice if m=1. It is also straight forward to usecombinations of semiconductor alloys, such as AlGaAs, to satisfy thesedesign requirements. For example, Al_(0.85)Ga_(0.15)As (n=3.07 at 850nm) and oxidized AlAs (=AlxOy with n=1.55) may be used to implement thephase filter. Other combinations of AlGaAs are also possible with somecompromise in the full set of requirements.

[0056] The addition of a half-wavelength (π/2) protective layer, themiddle cylinder 208 in FIG. 3, is highly beneficial not only because itprotects the VCSEL light-emitting portions (the bottom cylinder 206)while providing a base for the phase filter, but it also avoids the needto etch the VCSEL light-emitting portions. This acts to improve bothreliability and manufacturability.

[0057] While the general structures illustrated in FIGS. 2 and 3 arebeneficial, they are particularly useful when applied to a VCSEL that isdesigned to promote lasing in a higher-order operating mode that matchesthe optical characteristics of the phase filter and/or that tends tosuppress unwanted operating modes. That way, more optical power flowsthrough the phase filter. One general technique of promotinghigher-order mode lasing is to implement current crowding such that thecurrent into the active region is such that it favors a selectedhigher-order operating mode (m>0). Another general technique is tosuppress the fundamental and lowest-order modes, thus favoring thehigher-order operating modes. Yet another general technique is tosupport higher-order modes by using an oxide-confined cavity that issufficiently thick and located sufficiently close to the active regionso as to produce an effective confining index difference that a favorshigher-order modes.

[0058]FIG. 4 illustrates a VCSEL 300 that implements two of the threegeneral techniques described above that tend to favor higher-orderoperating modes and/or that suppress unwanted operating modes. As shown,the VCSEL 300 includes a metal contact 302 that includes M/2 slots 304that receive fingers 306 of the VCSEL 300, and that form ohmic contactmetal fingers 307. The ohmic contact metal fingers 307 are located anddimensioned to produce current flow into the active region of the VCSEL300 that enhances a selected higher-order operating mode.

[0059] The VCSEL 300 also includes an underlying oxidized film 309 formode confinement. That oxidized film 309 is located sufficiently nearthe active region (say below the third period of the top mirror) and issufficiently thick (having an optical path length of about λ/8 of theselected operating mode) that it produces an effective confining indexdifference (between the core and the cladding) such that the selectedoperating mode is operationally preferred. An effective confining indexdifference greater than 0.01 is beneficial. Furthermore, a large oxideaperture, say greater than 10 μm should be used as it not only enhancesthe selected higher-order mode but it also improves reliability.

[0060] While the VCSELs in FIGS. 2-4 are beneficial, another technique,referred to as “blazing,” can significantly increase the optical powerdirected into the desired far-field distribution. Referring now to FIG.5, blazing operates by providing a VCSEL 400 with additional phase shiftlevels, such as four, that optically interact to produce the desiredfar-field distribution. As shown, the VCSEL 400 includes a metal contact402 that surrounds pie-shaped filter slices 404, 406, 408, and 410. Thepie-shaped filter slices 404 provide 0 optical phase shifts, thepie-shaped filter slices 406 provide π/2 optical phase shifts, thepie-shaped filter slices 408 provide π optical phase shifts, and thepie-shaped filter slices 410 provide 3π/2 optical phase shifts.Furthermore, adjacent pie-shaped filter slices provide optical phaseshifts that differ by π/2. This enables fine “tuning” of the opticalperformance of the VCSEL 400 and enhanced filter performance.

[0061] While FIGS. 2 and 3 provide one technique, and FIGS. 4 and 5provide others, it should be clearly understood that these techniquesare not mutually exclusive. Indeed, the various techniques arecomplementary and subject to multiple improvements. For example, FIG. 6illustrates a VCSEL 600 that also promotes lasing in a predeterminedhigher-order mode. The VCSEL 600 includes a metal contact 602 thatincludes M/2 slots 604 and contact fingers 606. The VCSEL 600 alsoincludes an underlying (oxide or gain guide) aperture 608 and an inversefetch structure 609 within the underlying aperture 608. In the presentinvention, the term “fetch,” an acronym for “filter etch,” is defined asa pattern of etched films on a VCSEL's top surface intended to produce aspecific spatial variation in the reflectance of the top mirror. It isgenerally applied to promote lasing in a specific mode. Generally, afetch region includes an inner aperture region of higher reflectance andan outer region of lower reflectance in order to promote lasing in thefundamental mode. Likewise, the term “inverse fetch” refers to the casewhere the inner aperture region has lower reflectance and the outerregion has higher reflectance and tends to suppress the fundamental modewhile promoting higher order modes. Still referring to FIG. 6, insidethe inverse fetch region 609 are alternating pie-shaped filter slices610 and 612. The pie-shaped filter slices 610 provide π/4 optical phaseshifts, while the pie-shaped filter slices 612 provide 3π/4 opticalphase shifts. The inverse fetch structure creates a region of loweroptical reflectivity that inhibits lasing along the central axis of theactive region. The pie-shaped filter slices 610 provide π/4 opticalphase shifts, while the pie-shaped filter slices 612 provide 3π/4optical phase shifts. Outside the aperture 608 are phase shift segments616 and 618. The phase shift segments 616 provide π optical phaseshifts, while the phase shift segments 618 provide 2π optical phaseshifts. The phase shift segments 616 and 618 produce suitable phaseshifts to produce a desired intensity distribution in the far field.

[0062] To add an inverse fetch structure (a zone of lower reflectivity)in the center VCSEL for the suppression of the fundamental andlowest-order modes, a quarter-wave film may be added or subtracted fromthe VCSEL top mirror. The quarter-wave film may be made of a dielectricor a semiconductor material.

[0063] The foregoing has described using metal fingers located anddimensioned to produce current distributions in the VCSEL active regionthat support higher-order operating modes. This tailoring of currentdistributions can be performed in other ways. For example, FIG. 7illustrates a VCSEL 650 that also promotes lasing in a predeterminedhigher-order mode. The VCSEL 650 includes a metal contact 652, anunderlying (oxide or gain guide) aperture 654, and phase filter elements656 and 658. However, the VCSEL 650 further includes mode-matchedcurrent confining implants 660. The current confining implants 660,which are located in the top mirror, tailor the conductive paths intothe active region so as to enhance the selected higher-order mode.

[0064]FIG. 7 also illustrates alternatives to the use of inverse fetchstructures. The first alternative is an absorptive zone 664 thatattenuates optical beams along the central axis. Since optical beamsalong the central axis are predominately associated with the fundamentaland lower order modes, those modes are suppressed. Such absorptive zonescan be comprised of a thin film of titanium or by a heavy doping in thetop films. Rather than increasing the optical losses along the centralaxis to suppress the lower lasing modes, FIG. 7 also illustrates the useof a nonconductive central implant 666. That implant suppresses lowerorder mode lasing by reducing current flows that support such lasing.

[0065] The use of inverse fetch structures to inhibit lower order modeshas been previously discussed. A somewhat related idea can be used toenhance the selected higher-order operating mode. For example, FIG. 8illustrates a VCSEL 700 that includes thin antireflective zones 702. Theantireflective zones 702 are located at the field nulls of the selectedhigher-order operating mode. Thus, the antireflective zones 702 havelittle effect on the selected higher-order operating mode, but tend tosuppress undesired operating modes.

[0066]FIG. 8 also illustrates another method of enhancing the selectedhigher-order operating mode. Instead of using thin antireflective zones702 at the field nulls, either thin scattering zones 704 that arelocated at the field nulls or absorptive regions 706 (say thin films oftitanium) located at the field nulls can be used. Both methods havelittle effect on the selected higher-order operating mode, but tend tosuppress undesired modes. It should be noted that FIG. 8 shows threedifferent methods of suppressing undesired higher-order operating modes.In practice, using only one of the methods may be beneficial.

[0067] The embodiments and examples set forth herein are presented toexplain the present invention and its practical application and tothereby enable those skilled in the art to make and utilize theinvention. Those skilled in the art, however, will recognize that theforegoing description and examples have been presented for the purposeof illustration and example only. Other variations and modifications ofthe present invention will be apparent to those of skill in the art, andit is the intent of the appended claims that such variations andmodifications be covered. The description as set forth is not intendedto be exhaustive or to limit the scope of the invention. Manymodifications and variations are possible in light of the above teachingwithout departing from the spirit and scope of the following claims. Itis contemplated that the use of the present invention can involvecomponents having different characteristics. It is intended that thescope of the present invention be defined by the claims appended hereto,giving full cognizance to equivalents in all respects.

What is claimed is:
 1. A VCSEL device, comprising: a VCSEL structure forproducing higher-order mode laser light; a protective optical materiallayer on the VCSEL structure, wherein the protective optical materiallayer is for shifting the phase of the higher-order mode laser light bya half-wavelength (λ/2); and a phase filter on the protective opticalmaterial, the phase filter comprised of equal numbers of first phasefilter elements and second phase filter elements, wherein the first andsecond phase filter elements are for shifting the phase of thehigher-order mode laser light by phase shifts that differ by about(2m+1)(λ/2), wherein m=0, 1, 2, 3, . . . .
 2. A VCSEL device accordingto claim 1, wherein the first and second phase filter elements arepie-shaped.
 3. A VCSEL device according to claim 1, wherein the firstand second phase filter elements are comprised of different materials.4. A VCSEL device according to claim 3, wherein the first phase filterelements are comprised of air.
 5. A VCSEL device according to claim 4,wherein the thickness of the second phase filter elements isd=(2m+1)(λ/2)/(n₂−1), where n₂ is the refractive index of the materialthat comprises the second phase filter elements.
 6. A VCSEL deviceaccording to claim 1, wherein the second phase filter elements include amaterial selected from a group consisting of SiO₂, Si₃N₄, TiO₂,semiconductor materials, or combinations thereof.
 7. A VCSEL deviceaccording to claim 1, wherein the protective optical material layer isone of an oxide, a nitride, a semiconductor alloy lattice-matched to abase material of the VCSEL, and combinations thereof.
 8. A VCSEL,comprising: a VCSEL structure for producing higher-order mode laserlight, wherein the VCSEL structure has a central light axis; and a phasefilter on the VCSEL structure, the phase filter comprised of equalnumbers of first phase filter elements and second phase filter elements,wherein the first and second phase filter elements are for shifting thephase of the higher-order mode laser light by phase shifts that differby about (2m+1)(λ/2), wherein m=0, 1, 2, 3, . . . ; wherein the VCSELstructure includes a suppression structure along the central light axisthat suppresses lower-order mode laser light.
 9. A VCSEL according toclaim 8, wherein the suppression structure includes an inverse fetchfeature having a zone of lower reflectivity, wherein the zone of lowerreflectivity is disposed along the central light axis.
 10. A VCSELaccording to claim 9, wherein the inverse fetch feature is formed byadding quarter-wave films to part of the VCSEL structure along thecentral light axis and by removing quarter-wave films from part of theVCSEL structure along the central light axis.
 11. A VCSEL according toclaim 8, wherein the suppression structure includes a nonconductive zonealong the central light axis.
 12. A VCSEL according to claim 9, whereinthe nonconductive zone is formed by an implantation.
 13. A VCSELaccording to claim 8, wherein the suppression structure includes a lightabsorptive zone along the central light axis.
 14. A VCSEL according toclaim 13, wherein the light absorptive zone includes a thin film oftitanium.
 15. A VCSEL according to claim 13, wherein the absorptive zoneis formed by heavy doping of the VCSEL structure along the central lightaxis.
 16. A VCSEL, comprising: a VCSEL structure for producinghigher-order mode laser light from an active region, wherein the VCSELstructure includes an ohmic contact; and a phase filter on the VCSELstructure, the phase filter comprised of equal numbers of first phasefilter elements and second phase filter elements, wherein the first andsecond phase filter elements are for shifting the phase of thehigher-order mode laser light by phase shifts that differ by about(2m+1)(λ/2), wherein m=0, 1, 2, 3, . . . ; wherein the VCSEL structureincludes a current guide structure that causes current flow into theactive region that enhances a higher-order operating mode.
 17. A VCSELaccording to claim 16, wherein the current guide structure includesconductive fingers that extend from the ohmic contact.
 18. A VCSELaccording to claim 16, wherein the current guide structure includes animplant pattern in the VCSEL structure.
 19. A VCSEL, comprising: a VCSELstructure for producing a specific higher-order mode laser light from anactive region; and a phase filter on the VCSEL structure, the phasefilter comprised of equal numbers of first phase filter elements and ofsecond phase filter elements, wherein the first and second phase filterelements are for shifting the phase of the specific higher-order modelaser light by phase shifts that differ by about (2m+1)(λ/2), whereinm=0, 1, 2, 3, . . . ; wherein the VCSEL structure includes an opticalsuppression feature that suppresses operation at a mode other than thespecific higher-order mode.
 20. A VCSEL according to claim 19, whereinthe optical suppression feature includes an antireflective zone at anull field of the specific higher-order mode laser light.
 21. A VCSELaccording to claim 19, wherein the optical suppression feature includesa scattering zone at a null field of the specific higher-order modelaser light.
 22. A VCSEL according to claim 19, wherein the opticalsuppression feature includes a light absorptive zone at a null field ofthe specific higher-order mode laser light.
 23. A VCSEL according toclaim 22, wherein the light absorptive zone includes a thin film oftitanium.
 24. A VCSEL, comprising: a VCSEL structure for producing aspecific higher-order mode laser light from an active region; and aphase filter on the VCSEL structure, the phase filter comprised of equalnumbers of first phase filter elements and of second phase filterelements, wherein the first and second phase filter elements are forshifting the phase of the specific higher-order mode laser light byphase shifts that differ by about (2m+1)(λ/2), wherein m=0, 1, 2, 3, . .. ; wherein the VCSEL structure includes an oxide-confined cavity.
 25. AVCSEL according to claim 24, wherein the oxide-confined cavity has anoptical path length of approximately λ/8 of the specific higher-ordermode laser light.
 26. A VCSEL according to claim 25, wherein theoxide-confined cavity is located below the third period of a top mirrorin the VCSEL structure.
 27. A VCSEL according to claim 25, wherein theoxide-confined cavity has a diameter of greater than 10 μm.
 28. A VCSEL,comprising: a VCSEL structure for producing a specific higher-order modelaser light from an active region; and a phase filter on the VCSELstructure, the phase filter includes equal numbers of 1 to N^(th) phasefilter elements, wherein adjacent phase filter elements are for shiftingthe phase of the specific higher-order mode laser light by phase shiftsthat differ by about (2m+1)(?/N), wherein N=1, 2, 3, 4, . . . , andwherein m=0, 1, 2, 3, . . . .
 29. A VCSEL according to claim 28, whereineach phase filter element is pie-shaped.
 30. A VCSEL according to claim28, wherein the first phase filter elements are adjacent the second andfourth phase filter elements.
 31. A VCSEL device, comprising: a VCSELstructure for producing a laser light; a protective optical materiallayer on the VCSEL structure, wherein the protective optical materiallayer is for shifting the phase of the laser light by a half-wavelength(λ/2); and a phase filter on the protective optical material, the phasefilter including first phase filter elements and second phase filterelements, wherein the first and second phase filter elements are forshifting the phase of the laser light by phase shifts that differ byabout (2m+1)(λ/2), wherein m=0, 1, 2, 3, . . . .
 32. A VCSEL deviceaccording to claim 31, wherein the first and second phase filterelements are pie-shaped.
 33. A VCSEL device according to claim 31,wherein the first phase filter elements include a first material and thesecond phase filter elements include a second material, the first andsecond materials being different.
 34. A VCSEL device according to claim31, wherein the first phase filter elements include air.
 35. A VCSELdevice according to claim 31, wherein the second phase filter elementshave a thickness of d=(2m+1)(λ/2)/(n₂−1), where n₂ is the refractiveindex of the second material of the second phase filter elements.
 36. AVCSEL device according to claim 31, wherein the second phase filterelements include a material selected from the group consisting of SiO₂,Si₃N₄, TiO₂, semiconductor materials, and combinations thereof.
 37. AVCSEL device according to claim 31, wherein the protective opticalmaterial layer is one of an oxide, a nitride, a semiconductor alloylattice-matched to a base material of the VCSEL, and combinationsthereof.
 38. An optical device comprising: a VCSEL unit having an activeregion and producing an output laser light and a far field laser lighthaving a far field intensity distribution, wherein the VCSEL unit has acentral light axis; and a phase filter on the VCSEL unit, the phasefilter including first phase filter elements and second phase filterelements, the first and second phase filter elements shifting the phaseof the output laser light emitted from the active region of the VCSEL byphase shifts that differ by about (2m+1)(λ/2), wherein m=0, 1, 2, 3, . .. ; wherein the VCSEL unit includes a suppression structure along thecentral light axis that suppresses a mode of the output laser lightemitted from the active region that decreases an intensity of the farfield laser light having the far field intensity distribution.
 39. Anoptical device according to claim 38, wherein the suppression structureincludes an inverse fetch feature having a zone of lower reflectivity,wherein the zone of lower reflectivity is along the central light axis.40. An optical device according to claim 39, wherein the inverse fetchfeature is formed by adding a quarter-wave film to a part of the VCSELunit along the central light axis or removing the quarter-wave film froma part of the VCSEL unit along the central light axis.
 41. An opticaldevice according to claim 38, wherein the suppression structure includesa nonconductive zone along the central light axis.
 42. An optical deviceaccording to claim 39, wherein the nonconductive zone is formed by animplantation.
 43. An optical device according to claim 38, wherein thesuppression structure includes a light absorptive zone along the centrallight axis.
 44. An optical device according to claim 43, wherein thelight absorptive zone includes a thin film of titanium.
 45. An opticaldevice according to claim 43, wherein the light absorptive zone isformed by heavy doping of the VCSEL unit along the central light axis.46. An optical device comprising: a VCSEL unit having an active regionand producing an output laser light and a far field laser light having afar field intensity distribution, the VCSEL unit including an ohmiccontact; and a phase filter on the VCSEL unit, the phase filter havingfirst phase filter elements and second phase filter elements, the firstand second phase filter elements shifting the phase of the output laserlight emitted from the active region by phase shifts that differ byabout (2m+1)(λ/2), wherein m=0, 1, 2, 3, . . . ; wherein the VCSEL unitincludes a current guide, the current guide directing current flow intothe active region and enhancing an intensity of the far field laserlight having the far field intensity distribution.
 47. An optical deviceaccording to claim 46, wherein the current guide includes a conductiveprotrusion extending from the ohmic contact.
 48. An optical deviceaccording to claim 46, wherein the current guide includes an implantpattern in the VCSEL unit.
 49. An optical device comprising: a VCSELunit having an active region and producing an output laser light and afar field laser light having a far field intensity distribution; and aphase filter on the VCSEL unit, the phase filter having first phasefilter elements and of second phase filter elements, the first andsecond phase filter elements shifting the phase of the output laserlight emitted from the active region by phase shifts that differ byabout (2m+1)(λ/2), wherein m=0, 1, 2, 3, . . . ; wherein the VCSEL unithaving an optical suppression unit, the optical suppression unitsuppressing an operation at a mode other than the mode in accord withthe far field laser light having the far field intensity distribution.50. An optical device according to claim 49, wherein the opticalsuppression unit includes an antireflective zone at a null field of themode in accord with the far field laser light.
 51. An optical deviceaccording to claim 49, wherein the optical suppression unit includes ascattering zone at a null field of the mode in accord with the far fieldlaser light.
 52. An optical device according to claim 49, wherein theoptical suppression unit includes a light absorptive zone at a nullfield of the mode in accord with the far field laser light.
 53. Anoptical device according to claim 52, wherein the light absorptive zoneincludes a thin film of titanium.
 54. An optical device comprising: aVCSEL unit having an active region and producing an output laser lightand a far field laser light having a far field intensity distribution;and a phase filter on the VCSEL structure, the phase filter having firstphase filter elements and second phase filter elements, the first andsecond phase filter elements shifting the phase of the output laserlight emitted from the active region by phase shifts that differ byabout (2m+1)(λ/2), wherein m=0, 1, 2, 3, . . . ; wherein the VCSEL unitincludes an oxide-confined cavity.
 55. An optical device according toclaim 54, wherein the oxide-confined cavity is located below a thirdperiod of a top mirror in the VCSEL unit.
 56. An optical deviceaccording to claim 54, wherein the oxide-confined cavity has a diameterof greater than 10 μm.
 57. An optical device comprising: a VCSEL unithaving an active region and producing an output laser light and a farfield laser light having a far field intensity distribution; and a phasefilter on the VCSEL unit, the phase filter having 1 to N^(th) phasefilter elements, wherein adjacent phase filter elements of the 1 toN^(th) phase filter elements shift the phase of the output laser lightemitted from the active region by phase shifts that differ by about(2m+1)(λ/N), where N=1, 2, 3, 4, . . . , and where m=0, 1, 2, 3, . . . .58. An optical device according to claim 57, wherein each of the 1 toN^(th) phase filter elements has a pie-shape.
 59. An optical deviceaccording to claim 57, wherein the first phase filter elements areadjacent the second and fourth phase filter elements.
 60. An opticaldevice according to claim 57, wherein the number of the 1 to N^(th)phase filter elements is same.
 61. A VCSEL device according to claim 31,wherein a number of the first phase filter elements and a number of thesecond phase filter elements are same.
 62. An optical device accordingto claim 38, wherein a number of the first phase filter elements and anumber of the second phase filter elements are same.
 63. An opticaldevice according to claim 46, wherein a number of the first phase filterelements and a number of the second phase filter elements are same. 64.An optical device according to claim 49, wherein a number of the firstphase filter elements and a number of the second phase filter elementsare same.
 65. An optical device according to claim 54, wherein a numberof the first phase filter elements and a number of the second phasefilter elements are same.
 66. A VCSEL device, comprising: a VCSELstructure for producing a laser light; a protective optical means on theVCSEL structure for shifting the phase of the laser light by ahalf-wavelength (λ/2); and a phase filter on the protective opticalmaterial, the phase filter including first phase filter elements andsecond phase filter elements, wherein the first and second phase filterelements are for shifting the phase of the laser light by phase shiftsthat differ by about (2m+1)(λ/2), wherein m=0, 1, 2, 3, . . . .
 67. Anoptical device comprising: a VCSEL unit having an active region andproducing an output laser light and a far field laser light having a farfield intensity distribution, wherein the VCSEL unit has a central lightaxis; means for phase filtering the output laser light of the VCSELunit; and means for suppressing a mode of the output laser light emittedfrom the active region that decreases an intensity of the far fieldlaser light having the far field intensity distribution.
 68. A VCSELdevice, comprising: a VCSEL structure for producing lower-order modelaser light; a protective optical material layer on the VCSEL structure,wherein the protective optical material layer is for shifting the phaseof the higher-order mode laser light by a half-wavelength (λ/2); and aphase filter on the protective optical material, the phase filtercomprised of equal numbers of first phase filter elements and secondphase filter elements, wherein the first and second phase filterelements are for shifting the phase of the lower-order mode laser lightby phase shifts that differ by about (2m+1)(λ/2), wherein m=0, 1, 2, 3,. . . .
 69. A VCSEL, comprising: a VCSEL structure for producinglower-order mode laser light, wherein the VCSEL structure has a centrallight axis; and a phase filter on the VCSEL structure, the phase filtercomprised of equal numbers of first phase filter elements and secondphase filter elements, wherein the first and second phase filterelements are for shifting the phase of the lower-order mode laser lightby phase shifts that differ by about (2m+1)(λ/2), wherein m=0, 1, 2, 3,. . . ; wherein the VCSEL structure includes a suppression structurealong the central light axis that suppresses higher-order mode laserlight.
 70. A VCSEL, comprising: a VCSEL structure for producinglower-order mode laser light from an active region, wherein the VCSELstructure includes an ohmic contact; and a phase filter on the VCSELstructure, the phase filter comprised of equal numbers of first phasefilter elements and second phase filter elements, wherein the first andsecond phase filter elements are for shifting the phase of thelower-order mode laser light by phase shifts that differ by about(2m+1)(λ/2), wherein m=0, 1, 2, 3, . . . ; wherein the VCSEL structureincludes a current guide structure that causes current flow into theactive region that enhances lower-order operating mode.
 71. A VCSEL,comprising: a VCSEL structure for producing lower-order mode laser lightfrom an active region; and a phase filter on the VCSEL structure, thephase filter comprised of equal numbers of first phase filter elementsand of second phase filter elements, wherein the first and second phasefilter elements are for shifting the phase of the specific lower-ordermode laser light by phase shifts that differ by about (2m+1)(λ/2),wherein m=0, 1, 2, 3, . . . ; wherein the VCSEL structure includes anoptical suppression feature that suppresses operation at a mode otherthan the lower-order mode.
 72. A VCSEL, comprising: a VCSEL structurefor producing lower-order mode laser light from an active region; and aphase filter on the VCSEL structure, the phase filter comprised of equalnumbers of first phase filter elements and of second phase filterelements, wherein the first and second phase filter elements are forshifting the phase of the lower-order mode laser light by phase shiftsthat differ by about (2m+1)(λ/2), wherein m=0, 1, 2, 3, . . . ; whereinthe VCSEL structure includes an oxide-confined cavity.
 73. A VCSEL,comprising: a VCSEL structure for producing lower-order mode laser lightfrom an active region; and a phase filter on the VCSEL structure, thephase filter includes equal numbers of 1 to N^(th) phase filterelements, wherein adjacent phase filter elements are for shifting thephase of the lower-order mode laser light by phase shifts that differ byabout (2m+1)(λ/N), wherein N=1, 2, 3, 4, . . . , and wherein m=0, 1, 2,3, . . . .